The present invention relates to resins having gas barrier properties, curable barrier coating compositions utilizing such resins, and to packaging materials and/or containers including the barrier coatings. The barrier coatings substantially reduce the permeability of gases such as carbon dioxide and/or oxygen through packaging materials.
Plastics have found increasing use as replacements for glass and metal containers in packaging, especially of foods and beverages. The advantages of such plastic packaging includes lighter weight, decreased breakage (versus glass) and potentially lower costs. However, shortcomings in the gas-barrier properties of common packaging plastics, such as polyolefins, e.g., polyethylene and polypropylene, poly(ethylene terephthalate) and polycarbonates, present major disadvantages in the packaging of many foods and beverages. For example, many foods and beverages are sensitive to oxidation and must be protected from oxygen to prevent discoloration or other detrimental effects. Further, plastic beverage containers suffer comparative shelf-life problems versus glass or metal due to the loss of carbon dioxide or ingress of oxygen through the plastic container.
In addition to food applications, barrier coatings have utility for plastic medical ampoules and the like and for plastic fuel containers.
Numerous barrier coatings have been developed in the prior art, including barrier materials based on thermoplastic, crystalline resins such as vinylidene chloride or ethylene-vinyl alcohol. Each of these materials has drawbacks. Ethylene-vinyl alcohol-based polymers lose barrier properties upon exposure to water, and packages of this material cannot generally undergo retort, i.e., heating under pressurized steam for pasteurization without loss of barrier performance. Vinylidene chloride-based polymers have been recognized as having excellent gas-barrier properties, but preparation of such vinylidene chloride-based polymers must generally be done under high pressure. Further, since vinylidene chloride-based barrier materials include halogen atoms, the disposal of such materials via incineration poses environmental problems. In addition, both vinylidene chloride-based polymers and ethylene-vinyl alcohol based polymers exhibit loss of adhesion after undergoing retort.
U.S. Pat. No. 2,830,721 (Pinsky et al.) discloses a polyamine-polyepoxide barrier coating for plastic containers. The purpose is to reduce the permeation of organic solvents through polyethylene containers. For polymeric food and beverage containers, it would be desirable to provide barrier coatings that have lower oxygen and/or carbon dioxide permeabilities and less sensitivity to humidity than those disclosed in the Pinsky et al. patent.
Barrier coatings with very low permeability based on polyamine-polyepoxide resins with high amine nitrogen content are the subject of commonly owned U.S. Pat. No. 5,300,541, U.S. Pat. No. 5,006,381, and U.S. Pat. No. 5,008,137 (all to Nugent et al.). Preferred polyamines disclosed there are polyethylene polyamines such as tetraethylene pentamine, and the coatings of the preferred embodiments are characterized by amine nitrogen content of at least seven percent. These coatings have found commercial acceptance as barrier coatings for polymeric containers. Co-pending, commonly owned U.S. patent application Ser. No. 08/221,161, now abandoned discloses epoxy-amine barrier coatings containing metaxylylene diamine having improved performance under high humidity conditions, and in which the coatings are characterized by a cure ratio of at least 1.5 to 1 amine hydrogen equivalents to epoxy equivalent.
Metaxylylene diamine is known as a curing agent for polyepoxide resins. U.S. Pat. No. 4,605,765 (Miyamoto et al.) further discloses the use of the amine functional reaction product of metaxylylene diamine with epichorohydrin as a curing agent for polyepoxide resins.
The prior art (e.g., U.S. Pat. Nos. 4,908,272; 4,983,719; and 5,028,462) disclose reacting meta or para xylylenediamine with organic acids to form amides that serve as gas barrier layers. These amides are not liquids that can be used as coatings.